lib/boot_device: add RW boot device construct

The current boot device usage assumes read-only semantics to
the boot device. Any time someone wants to write to the
boot device a device-specific API is invoked such as SPI flash.
Instead, provide a mechanism to retrieve an object that can
be used to perform writes to the boot device. On systems where
the implementations are symmetric these devices can be treated
one-in-the-same. However, for x86 systems with memory mapped SPI
the read-only boot device provides different operations.

BUG=chrome-os-partner:55932

Change-Id: I0af324824f9e1a8e897c2453c36e865b59c4e004
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/16194
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

2 files changed, 34 insertions, 6 deletions

diff --git a/src/include/boot_device.h b/src/include/boot_device.h
index 3964a9508c..cc70442e1e 100644
--- a/src/include/boot_device.h
+++ b/src/include/boot_device.h
@@ -18,9 +18,18 @@
 
 #include <commonlib/region.h>
 
+/*
+ * Please note that the read-only boot device may not be coherent with
+ * the read-write boot device. Thus, mixing mmap() and writeat() is
+ * most likely not to work so don't rely on such semantics.
+ */
+
 /* Return the region_device for the read-only boot device. */
 const struct region_device *boot_device_ro(void);
 
+/* Return the region_device for the read-write boot device. */
+const struct region_device *boot_device_rw(void);
+
 /*
  * Create a sub-region of the read-only boot device.
  * Returns 0 on success, < 0 on error.
@@ -29,6 +38,13 @@ int boot_device_ro_subregion(const struct region *sub,
 				struct region_device *subrd);
 
 /*
+ * Create a sub-region of the read-write boot device.
+ * Returns 0 on success, < 0 on error.
+ */
+int boot_device_rw_subregion(const struct region *sub,
+				struct region_device *subrd);
+
+/*
  * Initialize the boot device. This may be called multiple times within
  * a stage so boot device implementations should account for this behavior.
  **/
diff --git a/src/lib/boot_device.c b/src/lib/boot_device.c
index 42fe5dfd35..e7968f4fa9 100644
--- a/src/lib/boot_device.c
+++ b/src/lib/boot_device.c
@@ -20,18 +20,30 @@ void __attribute__((weak)) boot_device_init(void)
 	/* Provide weak do-nothing init. */
 }
 
+static int boot_device_subregion(const struct region *sub,
+				struct region_device *subrd,
+				const struct region_device *parent)
+{
+	if (parent == NULL)
+		return -1;
+
+	return rdev_chain(subrd, parent, region_offset(sub), region_sz(sub));
+}
+
 int boot_device_ro_subregion(const struct region *sub,
 				struct region_device *subrd)
 {
-	const struct region_device *boot_dev;
-
 	/* Ensure boot device has been initialized at least once. */
 	boot_device_init();
 
-	boot_dev = boot_device_ro();
+	return boot_device_subregion(sub, subrd, boot_device_ro());
+}
 
-	if (boot_dev == NULL)
-		return -1;
+int boot_device_rw_subregion(const struct region *sub,
+				struct region_device *subrd)
+{
+	/* Ensure boot device has been initialized at least once. */
+	boot_device_init();
 
-	return rdev_chain(subrd, boot_dev, region_offset(sub), region_sz(sub));
+	return boot_device_subregion(sub, subrd, boot_device_rw());
 }